1. Field of the Invention
The present invention relates to a reflection-type micro display projection system. More particularly, the invention relates to a reflection-type micro display projection system having a polarization beam splitter with an optimized structure to increase optical efficiency and improve contrast of images.
2. Description of the Background Art
Recently, display systems have become light and thin while having a large screen. Particularly, a large-screen display system such as a projection TV system becomes important in the display field.
FIG. 1 shows the configuration of a conventional reflection-type micro display projection system. Referring to FIG. 1, the conventional reflection-type micro display projection system includes a lamp 1 emitting a light beam, an integrator 2 for changing the light beam emitted from the lamp such that the light beam has a uniform spatial distribution, a color wheel 3 for splitting the light beam output from the integrator 2 into time-sequential red, green and blue light beams, and first and second illumination lenses 4 for illuminating the split light beams to a reflection-type liquid crystal display panel 6. The system further includes a polarization beam splitter 5 for polarizing the light beams derived from the first and second illumination lenses 4, the reflection-type liquid crystal display panel 6 that reflects the polarized beams to output image information, a projection lens 7 for magnifying and projecting the image information, and a screen 8 for displaying an image corresponding to the image information magnified and projected by the projection lens 7.
The structure of the color wheel of the conventional reflection-type micro display projection system will now be explained in more detail.
FIG. 2 shows a filter structures of the color wheel. Referring to FIG. 2, the color wheel includes red, green and blue color filters 21, 22 and 23. The center of the color wheel is connected to a motor such that the color wheel is rotated. When the red filter 21 reaches the position of the integrator, the light beam of the color corresponding to the red wavelength band passes through the color wheel 3. The light beam of the color corresponding to the green wavelength band passes through the color wheel 3 when the green filter 22 is located at the position of the integrator. Furthermore, the light beam of the color corresponding to the blue wavelength band passes through the color wheel 3 when the blue filter 23 is located at the position of the integrator. In this manner, the light beams are time-sequentially arrived at the liquid crystal display panel. The light beams arrived at the liquid crystal display panel are transmitted or reflected in response to the type of the liquid crystal display panel to reach the projection lens. Then, the light beams pass through the projection lens to be imaged on the screen. While the light beam arrived at the liquid crystal display panel at an arbitrary moment is composed of one of red, green and blue beams, the beams arrived at the liquid crystal display panel can be recognized as a white beam when temporally summed up. The liquid crystal display panel can be divided into a single-panel type and a three-panel type based on the number of panels. Furthermore, the liquid crystal display panel can be divided into transmission-type and reflection-type liquid crystal display panels based on the method of converting light beams into image information.
In the meantime, the conserved quantity of light input to the reflection-type liquid crystal display panel in the conventional micro display projection system will now be explained with reference to FIG. 3. FIG. 3 is a diagram for explaining the operation of the polarization beam splitter used in the conventional reflection type micro display projection system. An f-number for defining the conserved quantity of light can be represented by Equation 1.F/#=1/(2×tan α)  [Equation 1]
Here, α represents an incident angle of the light beam output from the illumination lenses and input to the polarization beam splitter on the basis of the normal of the boundary face of the polarization beam splitter, and f-number is a value obtained by dividing the focal distance of the second illumination lens by its aperture. The Etendue that is the optical conserved physical quantity is represented using Equation 1 as follows.Etendue=π×S/(4×(F/#)2)  [Equation 2]
Here, S represents the area of the liquid crystal display panel and F# denotes the f-number of the illumination lens. The Etendue varies with the area of the liquid crystal display panel and the f-number of the illumination lens on a light path in the reflection-type micro display projection system. A variation in the Etendue in response to the area of the liquid crystal display panel and the f-number will now be explained in more detail with reference to FIGS. 4a and 4b. 
FIGS. 4a and 4b are graphs showing variations in the Etendue in response to the area of the liquid crystal display panel and the f-number. Specifically, FIG. 4a is a graph showing a variation in the Etendue in response to the diagonal length of the liquid crystal display panel and FIG. 4b is a graph showing a variation in the Etendue in response to the f-number.
The area of the liquid crystal display panel can be represented by the diagonal length of the panel. Thus, the Etendue depends on the diagonal length of the panel as shown in FIG. 4a. Referring to FIG. 4a, the Etendue increases as the diagonal length of the liquid crystal display panel increases when the f-umber of the illumination lens is 3.0. Referring to FIG. 4b, the Etendue decreases as the f-number of the illumination lens increases when the diagonal length of the liquid crystal display panel is 0.8 inch. That is, the Etendue is decided by the area of the liquid crystal display panel and the f-number.
In the meantime, in the micro display projection system that is a large-screen display, the liquid crystal display panel is fabricated on a wafer through a semiconductor device fabrication method. Accordingly, if the size of the liquid crystal display panel is reduced, the number of liquid crystal display panels capable of being formed on a wafer having a specific area can be increased to improve productivity and decrease the manufacturing cost.
To reduce the cost of the liquid crystal display panel of the micro display projection system in consideration of this characteristic of the liquid crystal display panel fabrication process, it is preferable to decrease the size of the liquid crystal display panel while maintaining the number of pixels formed in the liquid crystal display panel in consideration of resolution of images. Consequently, the pixel size of the liquid crystal display panel should be reduced in order to reducing the size of the liquid crystal display panel while maintaining the number of pixels.
When the pixel size of the liquid crystal display panel is decreased, the aperture ratio is reduced to deteriorate optical efficiency of the projection system in the case of the transmission-type liquid crystal display panel. In the case of the reflection-type liquid crystal display panel, the aperture ratio is maintained. However, even when the aperture ratio is maintained using the reflection-type liquid crystal display panel, the reduced size of the liquid crystal display panel decreases the Etendue to deteriorate optical efficiency. To compensate the deteriorated optical efficiency, the f-number should be reduced. However, this increases the incident angle of light beam input to the polarization beam splitter and degrades the performance of the polarization beam splitter. Consequently, the contrast of the projection system is deteriorated.